Secondary batteries such as lithium secondary batteries (typically, lithium ion batteries) and nickel hydride batteries have recently gained importance as power sources for vehicles that use electricity as a drive source or power sources installed on personal computers, portable terminals, or other electric products. In particular, lightweight lithium secondary batteries that make it possible to obtain a high energy density are expected to be advantageously used as high-output power sources for vehicles.
A lithium secondary battery is provided with electrodes of a configuration in which a material (electrode active material) capable of reversibly adsorbing and releasing lithium ions as charge carriers is held at a conductive member (electrode collector), and electrode active materials that make it possible to obtain a higher energy density and a higher output have been investigated. For example, lithium complex oxides such as lithium-cobalt complex oxide (LiCoO2), lithium-nickel complex oxide (LiNiO2), and lithium-manganese complex oxide (LiMn2O4), which have a lamellar rock salt structure, are known as positive electrode active materials constituting positive electrodes of lithium secondary batteries. Further, lithium-nickel-cobalt-manganese complex oxides in which atoms of nickel, cobalt, and manganese are disposed at metal sites other than those of lithium in the above-mentioned crystal structure have attracted attention as materials that excel in thermal stability and have a high energy density.
In a typical method for synthesizing the abovementioned lithium-nickel-cobalt-manganese complex oxides, powdered compounds serving as source materials are weighed to obtain the predetermined composition and mixed, and the mixture (that is, a starting source material) is fired. A synthesis method disclosed in Patent Literature 1 is an example of the conventional technique relating to such lithium-nickel-cobalt-manganese complex oxides. In this method the mixture of source materials (starting source material) is fired at a temperature of 500° C., and the temperature is then raised to 700° C. to 1000° C. for firing. Further, Patent Literature 2 discloses a method for synthesizing a lithium-nickel-cobalt-manganese complex oxide that is imparted with excellent output-recovery characteristic by adding Zr and Nb as other metal elements, which are different from lithium, nickel, cobalt, and manganese.